In recent years, there are very strong demands for smaller size and lighter weight of various electronic devices. In order to meet those demands, it is required to improve performance of a secondary battery as a power source, and hence development and improvement of various batteries have been carried out. For instance, a lithium ion secondary battery is a secondary battery that can realize highest voltage, highest energy density, and highest load bearing performance among existing battery types, and is being still improved intensely.
This lithium ion secondary battery generally includes a sheet-type internal electrode pair formed by laminating, via separators, sheet-type positive electrodes formed of a sheet-type positive electrode collector and a positive active material applied to a surface thereof, and sheet-type negative electrodes formed of a sheet-type negative electrode collector and a negative active material applied to a surface thereof, a battery case that covers the internal electrode pair in a sealed state and is filled with electrolyte, and a positive electrode terminal and a negative electrode terminal connecting respectively from the positive electrodes and the negative electrodes of the internal electrode pair in the battery case to a positive terminal and a negative terminal provided to the battery case. At the time of charging, lithium flows out of the positive active material of the positive electrode into the electrolyte as lithium ions, and the lithium ions enter the negative active material of the negative electrode. At the time of discharging, the lithium ions that have entered the negative active material are released into the electrolyte to return into the positive active material of the positive electrode again. Thus, charging and discharging are performed. This lithium ion secondary battery can achieve high energy density, and hence it is expected as a large capacity secondary battery to be used in the field of a hybrid vehicle (HV), an electric vehicle (EV), or the like, for example, and thus many developments and proposals have already been carried out.
For instance, as such a lithium ion secondary battery, there is proposed a battery having a sheath made of a laminated film, in which the internal electrode pair and the electrolyte are sealed (see, for example, Patent Literature 1).
Here, Patent Literature 1 discloses a sheet-type secondary battery including electricity generating elements such as positive electrodes, negative electrodes, and separators are housed and sealed in a sheathing film as a laminated sheath made of a metal-resin composite material. In order to prevent occurrence of wrinkles or the like in the sheathing film in a heat sealing process or the like, a sealing portion of the sheathing film is embossed.